Simplify the following expression: $k = \dfrac{1}{5n} - \dfrac{1}{4n}$
Solution: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $5n$ and $4n$ $\lcm(5n, 4n) = 20n$ $ k = \dfrac{4}{4} \cdot \dfrac{1}{5n} - \dfrac{5}{5} \cdot \dfrac{1}{4n} $ $k = \dfrac{4}{20n} - \dfrac{5}{20n}$ $k = \dfrac{4 -5}{20n}$ $k = \dfrac{-1}{20n}$